ANSWER
$51
Step-by-step explanation
Let the cost of one cookie be c.
Let the cost of one brownie be b.
3 cookies and 5 brownies cost $19. This means that:
3 * c + 5 * b = 19
3c + 5b = 19 ___(1)
2 cookies and 10 brownies cost $26. This means that
2 * c + 10 * b = 26
2c + 10b = 26 ____(2)
We have two simultaneous equations:
3c + 5b = 19 ___(1)
2c + 10b = 26 ____(2)
From (2):
2c = 26 - 10b
Divide through by 2:
c = 13 - 5b
Put this in (1):
3(13 - 5b) + 5b = 19
=> 39 - 15b + 5b = 19
Collect like terms:
-15b + 5b = 19 - 39
-10b = -20
Divide through by -10:
b = -20 / -10
b = $2
Remember that:
c = 13 - 5b
=> c = 13 - 5(2)
c = 13 - 10
c = $3
Therefore, one cookie costs $3 and one brownie costs $2.
Therefore, the cost of 13 cookies and 6 brownies is:
13 * 3 + 6 * 2
=> 39 + 12
=> $51
13 cookies and 6 brownies cost $51